The present invention relates to an electrical subsea cable with a conductor, an insulation and a sheath.
For the energy supply of electrical devices on the seafloor, subsea cables are known in the state of the art which contain one or several conductors with an insulation, typically made of cross-linked polyethylene, and an outer sheath. In cables for the medium high voltage range, the insulation comprises an inner screen adjacent to the surface of the conductor and a second screen on the outer surface of the insulation. The cable is generally armoured to be protected from damage which may be caused by outer impact forces, e.g. by fishing gears.
To prevent the blocking of oil pipelines by freezing oil components, especially when the flow is stopped, electrical heating systems are utilised. Patent application NO 1998 4235 describes an example of a heating system which can be used for pipelines on the sea floor. In this system, the metallic tube of the pipeline is electrically and thermally insulated and connected to a power supply which feeds a current through the tube. As the tube generally consists of ferromagnetic steel, an efficient heating is achieved with alternating current.
A subsea cable for the connection of the pipeline and the power supply may be squeezed during installation between the pipeline and hard objects, even if care is taken to avoid this situation. As the pipeline expands and contracts during operation, the cable is generally subject to stretching and sliding forces against the seabed. These problems are aggravated, if the pipeline spans valleys between reefs or boulders on the seabed. In addition, the cable has to be protected from impact forces which may be caused by fishing gears or falling objects.
A sufficient protection can be achieved with a steel armouring if the cable carries both the feeding and the return current. The above heating system, however, requires only a single conductor because the tube of the pipeline is used as conductor. In this case, an alternating current causes excessive electrical losses if a metal armour is applied to the cable. Another conceivable approach is an increase of the insulation thickness to provide the required protection. This would result in a disadvantageously high cable diameter.
It is therefore an object of the present invention to obviate these disadvantages and to develop an electrical subsea cable with a metal-free sheath which protects the cable from outer forces. It is a further object, to provide a cable with a small outer diameter.
According to the invention, the sheath comprises two polymer layers, wherein the outer layer has a mechanical hardness which is higher than the hardness of the insulation and wherein the hardness of the inner layer is lower than the hardness of the insulation.
A basic concept of the invention is a sheath which comprises at least two layers consisting of polymer materials with different mechanical properties. The outer layer is made of an elastic material with a high mechanical hardness, e.g. polyamide 12. An inner layer has a hardness which is lower than both the hardness of the insulation and the outer layer. For a cable with an insulation of cross-linked polyethylene, an inner layer of soft thermoplastic polyurethane elastomer is proposed. The rubber-like bedding ensures that damages of the insulation are avoided even if the outer layer is deformed due to outer forces. Consequently, the thickness of the outer layer can be reduced because small elastic deformations do not affect the insulation. A low total diameter of the subsea cable can be attained. Because the sheath is metal-free, electrical losses are avoided for a single conductor AC cable.
In a preferred embodiment of the invention, the sheath is extruded around the insulation. The inner layer is extruded on the outer surface of the insulation which will generally comprise an outer screen. It is conceivable, that the insulation or the screen is covered with a separating agent. At least one outer layer is extruded on the inner layer.
The sheath can be detachably fixed on the insulation. In this case, it is possible to insert the insulated conductor into the sheath when the cable is installed or remove it.
In an advantageous embodiment, the sheath has a slit parallel to the axis of the cable. This allows to insert the insulated conductors via the slit or equivalently to snap the sheath onto the insulation.
The cable may be subject to high axial forces, especially near the ends where it is fixed to other devices. To improve the resistance against axial forces, it is proposed that the sheath comprises fibres 45 (see, e.g., FIG. 3a) made from dielectric material, for example aramide. The fibres are disposed in or adjacent to the hard layer of the sheath to ensure an efficient transfer of forces. They can be limited to sections of the cable with especially high loading, e.g. near the ends, or the whole length of the sheath can be provided with fibres.
It is preferred that a gap is disposed between the sheath and the insulation. The gap is filled with water in the installed cable to avoid a compression of the sheath due to the hydrostatic pressure near the seabed. For this purpose, the gap is connected to the surroundings of the cable via the slit or other openings in the sheath or at the end of the cable. The water in the gap improves the cooling of the cable if sufficient circulation is allowed. Furthermore, the water enhances the damping properties of the inner layer if the outer layer is deformed due to impact forces.
Preferably the inner surface of the sheath is grooved wherein the grooves constitute a part of the gap between sheath and insulation. Protrusions between the grooves support the insulated cable core in the centre of the sheath. The grooves in the inner layer provide an additional damping compared to a compact inner layer.
To avoid capacitive charging of an outer screen on the insulation, metallic drain conductors can be used, e.g. a copper tape or copper wires. If the insulation of the cable is in contact with surrounding water, the insulation can comprise a semiconducting layer, e.g. semiconducting cross-linked PE, which encloses the outer screen instead of drain conductors. This avoids electrical losses due to metallic drain conductors.
In a preferred embodiment, the cable is fixed along a support, for example a pipeline for fluids. Suitable fixing means are straps.
A round cross-section of the cable is preferred when it is laid separately on the seabed. If the cable is fixed to a pipeline or a similar support, a surface of the sheath which is shaped complementary to the support is advantageous.
The proposed cable is suitable for use in an electrical heating system for an insulated metallic tube, especially a pipeline, wherein a current is fed through the wall of the tube. The electrical insulation generally serves simultaneously as thermal insulation and can consist for example of polypropylene. The tube is preferably made of a ferromagnetic material like steel and heated by an alternating current in the range of several 100 A to several kA and voltages between 1 and several ten kV, depending on the cross-section and the required heating power. The cable connects the tube to a power supply which can be an armoured rising cable from the seafloor to the surface where it is connected to a feeding unit. The heating system may either be closed, i.e. isolated from the surrounding sea. Alternatively, open systems may be used wherein a part of the current flows through the seawater.
The foregoing and other objects, features and advantages of the present invention will become more apparent in the following detailed description of preferred embodiments as illustrated in the accompanying drawings.